1. Field of the Invention
The present invention relates generally to blast nozzles and a process for removing adherent material such as paint, scale, dirt, grease and the like from solid surfaces with abrasive particles propelled by air. In particular, the present invention is directed to a novel blast nozzle which is shaped to provide uniform flow of blast media therethrough and is particularly useful in blasting with a friable and relatively soft abrasive media such as sodium bicarbonate.
2. Description of the Prior Art
In order to clean a solid surface so that such surface can again be coated such as, for example, to preserve metal against deterioration, remove graffiti from stone or simply to degrease a solid surface such as surfaces contacting food or building structures which contain food serving or food processing operations, it has become common practice to use an abrasive blasting technique wherein abrasive particles are propelled by a high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. Various abrasive blasting techniques have been utilized to remove the coatings, grease and the like from solid surfaces. Thus, blasting techniques comprising dry blasting which involves directing the abrasive particles to a surface by means of pressurized air typically ranging from 30 to 150 psi, wet blasting in which the abrasive blast media is directed to the surface by a highly pressurized stream of water typically 3,000 psi and above, multi-step processes comprising dry or wet blasting and a mechanical technique such as sanding, chipping, etc. and a single step process in which both air and water are utilized either in combination at high pressures to propel the abrasive blast media to the surface as disclosed in U.S. Pat. No. 4,817,342, or in combination with relatively low pressure water used as a dust control agent or to control substrate damage have been used. Water for dust control has been mixed with the air either internally in the blast nozzle or at the targeted surface to be cleaned and such latter process, although primarily a dry blasting technique, is considered wet blasting inasmuch as media recovery and clean up is substantially different from that utilized in a purely dry blasting operation.
A typical dry blasting apparatus as well as a wet blasting apparatus which utilizes highly pressurized air to entrain, carry and direct the abrasive blast media to the solid surface to be treated and low pressure water for dust control comprises a dispensing portion in which the blast media typically contained in a storage tank is entrained in highly pressurized air, a flexible hose which carries the air/blast media mixture to the blast nozzle and which allows the operator to move the blast nozzle relative to the surface to be cleaned and the blast nozzle which accelerates the abrasive blast media and directs same into contact with the surface to be treated. The blast nozzle is typically hand-held by the operator and moved relative to the targeted surface so as to direct the abrasive blast media across the entire surface to be treated.
The blast media or abrasive particles most widely used for blasting surfaces to remove adherent material therefrom is sand. Sand is a hard abrasive which is very useful in removing adherent materials such as paint, scale and other materials from metal surfaces such as steel. While sand is a most useful abrasive for each type of blasting technique, there are disadvantages in using sand as a blast media. For one, sand, i.e., silica, is friable and upon hitting a metal surface will break into minute particles which are small enough to enter the lungs. These minute silica particles pose a substantial health hazard. Additionally, much effort is needed to remove the sand from the surrounding area after completion of blasting. Still another disadvantage is the hardness of sand itself. Thus, sand cannot readily be used as an abrasive to remove coatings from relatively soft metals such as aluminum or any other soft substrate such as plastic, plastic composite structures, concrete or wood, as such relatively soft substrates can be excessively damaged by the abrasiveness of sand. Moreover, sand cannot be used around moving parts of machinery inasmuch as the sand particles can enter bearing surfaces and the like.
An alternative to non-soluble blast media such as sand, in particular, for removing adherent coatings from relatively soft substrates such as softer metals as aluminum, composite surfaces, plastics, concrete and the like is sodium bicarbonate. While sodium bicarbonate is softer than sand, it is sufficiently hard to remove coatings from aluminum surfaces and as well remove other coatings including paint, dirt, and grease from non-metallic surfaces without harming the substrate surface. Sodium bicarbonate is not harmful to the environment and is most advantageously water soluble such that the particles which remain subsequent to blasting can be simply washed away without yielding environmental harm. Unfortunately, sodium bicarbonate, typically used as particles having average diameters of from about 50 to 1,000 microns, is even more friable than sand and breaks into smaller particles as it traverses the flexible supply hose which carries the blast media and pressurized air to the blast nozzle and, as well, breaks into pieces as the blast media comes into contact with the internal surfaces of the blast nozzle prior to being propelled to the target surface.
Sodium bicarbonate blast media has been propelled by a standard round nozzle which comprises a converging hollow conical inlet section, a venturi throat and a contiguous diverging hollow conical outlet section and which is typically used for blasting with sand. As above described, it has been found that the relatively light sodium bicarbonate blast media loses a substantial portion of its effectiveness due to the break up of the individual particles in the round nozzle. Moreover, it has been found that the individual particles of sodium bicarbonate are rounded during travel through the blast nozzle such that the sharp cutting edges are broken off, likely reducing the cutting action and effectiveness of the media for contaminant removal from the substrate. The conical shape of the converging and diverging sections of the round nozzle is believed to be one source of these problems. Thus, as the sodium bicarbonate blast media enters the nozzle from the supply hose and converges toward the venturi orifice and then expands subsequent to the venturi orifice, the individual particles of the blast media are believed to be directed not only in the longitudinal direction toward and away from the venturi orifice, but radially, literally bouncing along all of the surfaces of the conical sections. As the individual particles of sodium bicarbonate lose mass within the blast nozzle and, are not optimally accelerated through the nozzle due to the turbulent flow of misdirected particles, there consequently results a degradation in the productivity of the blasting operation. Accordingly, there is a need to provide a blast nozzle which can be used for blasting with sodium bicarbonate as the blast media and which will not yield the substantial loss of productivity found when using a round nozzle.
It would also be useful to change the conditions of blasting without having to use a different blast nozzle. Thus, standard round nozzles and other blast nozzles include venturi sections to accelerate the blast media from the nozzle that are passage-ways typically machined or cast such as in metal blast nozzles or pressed or molded as in ceramic nozzles and, thus, cannot be adjusted to accommodate different densities of blast media or changing on-site conditions. Inefficiencies are simply tolerated or a new nozzle with different properties is provided.
An attempt has been made to tailor a blast nozzle for use in blasting with abrasive media which is softer than sand such as plastic pellets. This blast nozzle included a converging section, a throat and a diverging or expansion section in the shape of a fan which directed the blast media to the surface as a fan shaped stream of particles. The inventor found that the prototype fan nozzle was extremely inefficient in blasting with sodium bicarbonate. It is now believed that the inefficiencies that were found resulted from (1) a converging or inlet section which was not sufficiently long, it being slightly less than twice the diameter of the inlet which resulted in an excessively steep convergence and consequent turbulence in the blast media/air stream through the nozzle, (2) a rectangular venturi orifice which was wider than the diameter of the supply hose resulting in simultaneous expansion and convergence of the blast media/air stream and additional turbulence and (3) it could not be adjusted on-site inasmuch as the converging section was machined within the metal structure which formed the prototype nozzle. Thus, the geometry of the prototype blast nozzle is now believed to have resulted in a substantial amount of turbulent flow causing excessive contact of the particles of blast media with the walls of the nozzle. As found in using the round nozzle, the turbulent flow resulted in an uneven outlet flow and loss of velocity and mass with respect to the individual abrasive particles.
It is a primary objective of the present invention to provide a blast nozzle which is useful in blasting to remove contaminants, such as rust, coatings, dirt, grease, etc. from a surface utilizing sodium bicarbonate as the blast media.
Another objective of the present invention is to provide a blast nozzle which has readily adjustable geometry to maintain optimum velocity of the blast media from the outlet of the blast nozzle, regardless of blast media type, size or density or changing on-site conditions.